1. Field of the Invention
This invention generally relates to integrated circuit fabrication and, more particularly, to a system and method for smoothing surface protrusions in the surface of laser annealed semiconductor films.
2. Description of the Related Art
When forming thin film transistors (TFTs) for use in liquid crystal display (LCD) or other microelectronic circuits, the location of transistors channel regions, the orientation of regular structured polycrystalline silicon (poly-Si) or single-grain-crystalline silicon, and the surface roughness are important issues. This poly-Si material can be used as the active layer of poly-Si TFTs in the fabrication of active-matrix (AM) backplanes. Such backplanes can be used in the fabrication of AM LCDs and can be also combined with other display technologies, such as organic light-emitting diode (OLED) displays.
Poly-Si material is typically formed by the crystallization of initially deposited amorphous Si (a-Si) films. This process can be accomplished via solid-phase-crystallization (SPC), for example, by annealing the a-Si films in a furnace at appropriate temperature and for sufficiently long time. Alternatively, laser annealing can also be used to achieve the phase transformation.
Conventionally, crystallization techniques are applied to a substrate in such a manner as to yield uniform poly-Si film quality throughout the substrate area. In other words, there is no spatial quality differentiation over the area of the substrate. The most important reason for this end result is the inability of conventional methods to achieve such quality differentiation. For example, when a-Si film is annealed in a furnace or by rapid-thermal-annealing, all of the film is exposed to the same temperature, resulting in the same quality of poly-Si material. In the case of conventional laser annealing, some differentiation is possible, but the price, in terms of loss of throughput, is very high for the modest performance gains realized.
Recently, a new laser annealing technique has been developed that allows for significant flexibility in the process techniques, permitting controlled variation in resulting film microstructure. This technique relies on lateral growth of Si grains using very narrow laser beams, that are generated by passing a laser beam through a beam-shaping mask, and projecting the, image of the mask to the film that is being annealed. The method is called Laser-Induced Lateral Crystallization (LILaC), sequential lateral solidification (SLS), or SLS/LILAC. More conventional solid state or continuous laser annealing processes can be differentiated from LILAC processes by their use of relatively long pulse durations and wide beam widths (or mask apertures). The poly-Si material crystallized by the continuous laser annealing method consists of a large density of grains, and each grain is surrounded by grain boundary. The size of grains are typically ˜1 micron (μm). But the typical channel length of TFT is 2˜30 microns, so it is inevitable that channel regions of TFT contain several grain boundaries. These grain boundaries act as electron and hole traps, and degrade the TFT characteristics and reliability. The LILAC process can form larger grain lengths between grain boundaries.
FIG. 1 is a diagram illustrating the LILaC process (prior art). The initially amorphous silicon film is irradiated by a very narrow laser beamlet, with typical widths of a few microns (i.e. 3-5 μm). Such small beamlets are formed by passing the original laser beam through a mask that has open spaces or apertures (see FIG. 2), and projecting the beamlets onto the surface of the annealed Si-film.
FIG. 2 is a conventional laser annealing mask (prior art). Returning to FIG. 1, the sequence of images 1 through 4 illustrates the growth of long silicon grains. A step-and-repeat approach is used. The shaped laser “beamlet” (indicated by the 2 parallel, heavy black lines) irradiates the film and then steps by a distance smaller than half of the width of the slit. As a result of this deliberate advancement of each beamlet, grains are allowed to grow laterally from the crystal seeds of the poly-Si material formed in the previous step. This is equivalent to laterally “pulling” the crystals, as in zone-melting-crystallization (ZMR) method or other similar processes. As a result, the crystal tends to attain very high quality along the “pulling” direction, in the direction of the advancing beamlets. This process occurs simultaneously at each slit on the mask, allowing for rapid crystallization of the area covered by the projection of the mask on the substrate. Once this area is crystallized, the substrate moves to a new (unannealed) location and the process is repeated.
FIG. 3 is a pictorial representation of a system to accomplish the optical projection and the step-and repeat process (prior art). The LILaC process has the potential for creating intentional spatial variations in the quality of the poly-Si material. Such intentional variations can be beneficial for applications where multiple components are integrated on a display, where each component has different specifications and material performance requirements.
Some poly-Si materials formed through the LILaC process have a highly periodical microstructure, where crystal bands of specific width are separated by high-angle grain boundaries. Within the crystal bands, low-angle boundaries are observed with a frequency of occurrence dependent upon certain specifics of the crystallization process, such as film thickness, laser fluence (energy density), pulse duration, and the like. TFTs fabricated on such poly-Si films demonstrate very good characteristics, as long as the direction of conduction is parallel to the direction of the in-crystal low-angle boundaries.
TFTs with greater electron mobility can be fabricated if the substrate crystallization characteristics can be made more isotropic. In other words, the TFT performance depends upon the angle between the main crystalline growth direction, the direction parallel to the laser scanning axis, and the TFT channel. This is due to the formation of sub-boundaries within the crystal domains. The surface roughness at the “hard” grain boundaries, at the edges of the crystal bands/domains, can be significant. This surface roughness prohibits the reduction of the gate insulator thickness, which is one critical step for scaling down the device geometry for future applications. Further, not all of these processes can be location controlled. Therefore, by chance only, depending upon the relative size of the crystal domain and the TFT channel length, certain TFTs will not include grain-boundaries in their active area (channel), whereas other TFTs will include one or more boundaries in their active areas. This kind of non-uniformity is highly detrimental for critical-application TFTs where uniformity of characteristics is more essential than absolute performance.
FIG. 4 is partial cross-sectional view of FIG. 1 illustrating the surface topography of laser-irradiated domains (prior art). After the completion of the lateral growth, the two crystal fronts meet at the center of the domain where they form a “boundary” between the two crystal regions developing from each opposing edge of the domain. As a result of the grain boundary formation, a “ridge” develops at the surface of the film at the boundary, corresponding to the planned congruence of the two crystal fronts. Since the substrate steps under the beam a distance of d, where d is less than L/2, the ridge is irradiated is a subsequent shot. This ridge remelts and locally planarizes. However, as part of the same process, another ridge is formed at a new location. Therefore, the ridge location will “march” across the substrate in response to the scans under the beam.
For single-crystal regions or directionally solidified material, devices can be fabricated on crystallized regions that have no protrusions. But for large-grained polycrystalline SLS material (e.g., material produced via the 2-shot or 2N processes), there can be a large number of surface protrusions that can adversely affect the device performance and limit the thickness of gate insulator that can be used. Thus, it is imperative that these protrusions be eliminated for optimal device performance.
If the angle of rotation between the lattice mismatch on the two sides of the boundary is less than approximately 15 degrees, the boundary is considered to be a low-angle boundary. An angle of rotation between 15 and 90 degrees is considered to be a high-angle boundary. Electron mobility between high-angle boundaries is impaired, while mobility between low-angle boundaries is usually insignificant. The step-and-repeat annealing typically promotes low-angle boundaries. However, the film regions corresponding to the mask edges, not being subject to the step-and-repeat process, are likely to form high-angle boundaries.
One embodiment of the SLS/LILaC process involves the use of a large array of narrow slits that simultaneously melt and solidify the Si thin film in such a way as to fully crystallize the entire film after two passes that are stitched together. The drawback to such an approach is that due to volume expansion of the Si material during solidification, a large peak appears in the center of each irradiated region with a magnitude approximately equal to that of the film thickness. This peak-to-valley roughness can be detrimental to the characteristics of devices subsequently fabricated on the thin film. One way to eliminate or reduce the magnitude of this surface roughness is to partially melt the Si thin film and cause the mass to redistribute itself to reduce the surface tension of the liquid material. A mask that uses diffractive optics can be used to create a homogenous beam with a reduced transmission in the energy density at the sample plane. This homogenous beam then “flood” irradiates the sample and induces partial melting of the film.
It would be advantageous if the surface of laser annealed films could be made more smooth, so that thinner overlying films could be formed.
It would be advantageous if the post annealing surface smoothing process could be further refined to increase throughput and to reduce processing times.